This invention relates to an improved method for proportioning concrete materials in order to obtain an optimum proportion of the concrete mix components. The invention includes also the evaluation of aggregate and the optimum mix proportioning of aggregate to be incorporated in concrete.
In conventional concrete mix design procedures, various assumptions are made about the characteristics of the aggregates that will be incorporated in the concrete mix. When characterizing an aggregate, for instance, the things that influence the way the aggregate performs in concrete are aggregate size, aggregate shape, and aggregate surface texture as well as any deleterious substances or materials present, or adhering to the aggregates. These properties influence the way that the aggregate packs together in a matrix, the amount of bond potential between aggregate particles and cement paste, and the water demand of the concrete to achieve a requisite fresh concrete workability.
Existing methods for measuring the size of a given aggregate include grading the aggregate particles by passing a sample of aggregates over a standard series of sieves that have square apertures of a specific size. The mass (weight) of materials retained on a given sieve size is recorded and expressed as a percentage of the total mass (weight) of the sample being sieved or percent retained. Typically, the result of each of these tests as a percent retained is subtracted from the former sieves"" passing starting with the largest sieve through which 100% passes. This test is termed a percent passing grading or gradation or sieve analysis for the entire sample. The main drawback of this sieve analysis or grading test is that the apertures used in the standard sieves or screens are specified as square. The measured dimension of the aggregate that passes through the sieve is the middle size dimension or axis. A sieve does not measure the least or the greatest dimension. Because aggregate particles are irregular in shape, it is probable that the same square aperture will allow a flat or elongated particle to pass through the diagonal of the aperture and give a false measurement of the median dimension that is supposed to be measured.
Unfortunately, most existing mix design procedures utilize only the grading or sieve analysis data to characterize the size distribution of aggregates. This results in an inaccurate measure of the particle size distribution, because of the multitude of shapes that can give an xe2x80x9capparentxe2x80x9d particle size. This inaccuracy in measurement is also seen in actual test running and batch analysis. For instance, two different aggregates having identical gradations can often yield different qualities of concrete. In summary, therefore, the sieve analysis is not the most effective for characterizing the size of aggregates for a concrete mixture.
Particle shape is another important characteristic to take into account when looking at proportioning the aggregates correctly in a concrete mix design. Currently, test methods that are used to determine particle shape are limited to a two dimensional test to simply conclude whether or not the aggregate is flat or elongated. This shape determination (measuring length and width) is made by expressing the percentage of mass of the sample that does not fit into a given ratio of the total sample. For example, if a permissible range depends on a percent by weight which must be less than a ratio of 5 to 1 (maximum dimension to minimum dimension), then the test does not accurately express the degree of misshapenness. For instance, in the example of an acceptable ratio of a percent by weight of less than 5 to 1, all the particles could have a ratio of 4.99 to 1.0 and therefore be considered a 100% pass rate while a sample having all particles with a ratio of 5.01 to 1.0 would have a 100% fail rate. In reality, these materials would most likely behave identically in a concrete mix.
Aggregate surface area and texture are also important attributes for determining the performance of a concrete mixture in the plastic and hardened states. The impact of these attributes on the concrete increases as the nominal size of the aggregate decreases. In the finer aggregate size fraction however, there is a critical specific surface value where this relationship diminishes. When the fine aggregate particles approach the size of the cement particles (less than 150 microns), the apparent influence of the high specific surface is minimized.
Also, in conventional test methods, a characterization of the fine aggregate portion is made using a value called the fineness modulus. The fineness modulus, or FM, for the fine aggregate portion of the mix is the sum of the percentage of the materials cumulatively retained, beginning at about the 9.5 mm sieve down to and including the 100 mesh sieve or 150 micron size. It is possible to have different gradations representing the same FM value, and hence this value can be misleading as to the representation of the overall particle size distribution of fine aggregate material. Especially important is the use of the fine aggregate FM to determine the volume of the coarse aggregate to be used in the mix.
Also, current concrete mix proportioning relates to the way various densities and voids calculations are made with respect to given aggregate samples. These samples are typically xe2x80x9croddedxe2x80x9d whereby the sample is compacted during the mix proportioning calculation process. This is an unrealistic method, because the aggregate that is incorporated into the concrete will not have the same density as an artificially compacted sample. The rodding technique, therefore, renders the entire mix proportioning process as potentially inaccurate.
Accordingly, it is object of the present invention to overcome the foregoing drawbacks and provide a method for mixing concrete to obtain an optimum proportion of the concrete mix components including an optimum mix proportioning of the aggregate to be incorporated in the concrete.
In one embodiment, the method for selecting proportions of aggregate for incorporation into concrete comprises a series of steps. A maximum aggregate size for the specific application is determined. A sample of the maximum aggregate size is loose poured into a container and the percentage voids in the sample is calculated. An amount of smaller sized aggregate equal to approximately the volume of the percentage voids calculated is then added to the sample until a reduced volume of voids in the mixed sample is achieved. The amounts of aggregates determined to have the reduced volume of voids in the mix sample is then used in a concrete mixture. Also, the method may include repeating the steps of calculating the percentage voids and adding smaller sized aggregate to the mix sample. Further, cementitious material and water can be added to the aggregate to form a concrete mixture. Additionally, the maximum coarse aggregate size should be no greater than one-third of the lift height of the concrete to be poured, one-fourth of the nominal diameter of the pumping hose, and one half of the nominal spacing between reinforcement bars.
In another embodiment of the invention, a sample of aggregate may be evaluated by selecting a representative sample of the aggregate. For each stone in the sample, the greatest dimension of the stone is measured and recorded. Then, a template having different sized apertures cut into it is provided. Each stone in the sample is then passed through the apertures in the template until the stone is retained on the template. The measurement of the aperture on which the stone is retained is recorded. The recorded information is then used to calculate the volume of the aggregate stones in the sample.
In a still further embodiment of the present invention, a method of selecting aggregate for incorporation into a concrete mixture comprises only using an amount of aggregate whose total ultra-fines percentage content complies with the formula (Axc3x97y/3.15) less than (xe2x88x92300xc3x97B)+2000. In this formula, A equals the cementitious materials content of the concrete mixture in kilograms per cubic meter, B equals the total ultra-fines percentage content in the aggregate to be used in the concrete mixture by volume, and y equals the density of the cementitious material.
In a still further embodiment, a method of selecting aggregate for incorporation into a concrete mixture comprises using only an amount of aggregate whose total ultra-fines percentage content and methylene blue test values comply with the following formula: 0.08xe2x89xa7Cxc3x97D. In the foregoing formula, C equals the total ultra-fines percentage content by volume in decimal form and D equals the methylene blue test value of the aggregate to be used in the concrete mixture.